Insulated wires used as internal wirings of electronic machinery are required to have a flame-retardant property in order to avoid spread of fire through electrical wires in fire accident of the machinery and others. Standards for the flame-retardant properties of internal wiring materials are defined by, for example, UL758 standards of the U.S. and others.
Meanwhile, in Europe where railroad vehicle networks have been developed, adoption of regional uniform standards that are called EN standards (European standards) has been spread, and it has been required to use electrical wires and cables each having a heat resistance property, a flame-retardant property, a hydrolysis resistance property, an abrasion resistance property and a low smoke-producing property, and each using a halogen-free material not containing halogen for a covering material.
As the halogen-free material, polyolefin-based resins such as polyethylene and polypropylene are cited. However, usage of only the polyolefin-based resin is insufficient for the flame-retardant property, and therefore, a method of adding a halogen-free flame retardant is used. As the halogen-free flame retardant, metallic hydroxides such as magnesium hydroxide and aluminum hydroxide are cited (Japanese Patent Application Laid-Open Publication No. 2000-109638 (Patent Document 1)).
A cross-linked material obtained by cross-linking of halogen-free cross-linking resin composition has the flame-retardant property and a mechanical property, and besides, is excellent in a fuel resistance property, a cold resistance property and a room-temperature storable property. Therefore, it is known that such a cross-linked material can be suitably used for an insulating layer of an insulated wire and a sheath (Japanese Patent Application Laid-Open Publication No. 2015-21120 (Patent Document 2)).